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Slide 1

MAGMA = molten rock below the surface Only called LAVA once it has come out of the top of a volcano Magma is a mixture of molten rock, volatiles and solids. It may also contain suspended crystals and dissolved gas and sometimes gas bubbles. We make the assumption that the mantle is homogenous (the same) BUT not all igneous rocks are the same, i.e. cooling rates dictates crystal size, thereby appearance, and magma differs due to the nature of tectonics. Silica is the key ingredient in determining how a volcano erupts and its shape.More Silica = more VISCOUS lava more gas trapped, lava flows less common but more pyroclastic flows as eruptions are explosive not effusive, produces steep-sided volcanoes All magma does contain dissolved gases but as they rise to the surface to erupt, some gases are released quietly and others explosively. Thin fluid lava (described as not viscous) permit gases to escape with ease whereas thick lava (highly viscous) prevents release of gases so pressure builds up, until an explosive eruption allows the gases t o escape. Silica content varies in volcanoes around the world.......…read more

Slide 2

Why are there such differences in volcanoes and igneous rocks?Magma `evolves' as it rises 3. Silica is the last mineraland as some plate boundaries to form and the first toare thicker than others, melt, therefore anymagma evolves to a differing volcanoes above aextent around the world subduction zone will- The thicker the plate the already be enriched in silica more evolved the magma 2. As this cools as it travels closer to the surface, more and more crystals form, further enriching the melt with silica... 1. At about 900 degrees Celsius, heavy non-silicates turn into solid crystal within the melt, becoming heavier= FRACTIONAL CRYSTALLISATION (denser) and so sink to leave/ MAGMATIC DIFFERENTIATION the melt enriched in silica…read more

Slide 6

Volcano Shapes...Fissures:Most commonly found around spreading ridges andnormally basaltic so lots found in Iceland and Hawaii i.e.1973 eruption of Heimaey. This was also responsible for theflood basalts of Fingals Cave and Giants Causeway.Eruptions like this form plateaus as the lava has lowviscosity so flow rapidly and far away from its source.ROCK TYPE LOCATION ERUPTIONSBasaltic Rifts/early divergent Gentle, persistent marginsShield:Basaltic rock forming gently sloping cones from layers ofless viscous lava. Form some of the largest structure on theEarth i.e. Mauna Loa in Hawaii is largest volcano in theworld.ROCK TYPE LOCATION ERUPTIONSBasaltic Hotspots and Oceanic vs. Gentle, persistent Oceanic convergenceComposite:Most common type found on land. Created by layers of ashfrom initial explosive phases of eruptions and subsequentlayers of lava from main phase. Lava tends to be moreacidic so more viscous, therefore does not flow as far =steep sides. Typical examples are Vesuvius, Etna andPopocatepetl.ROCK TYPE LOCATION ERUPTIONSAndesitic Convergent margins Explosive, unpredictable…read more